Climate change is recognized as a significant problem that has received considerable attention. As a result of worldwide generation of energy from fossil fuels, large amounts of greenhouse gasses are accumulating in our atmosphere. Many experts believe that if something is not done soon to slow or even reverse this accumulation, our climate and the world in which we live will suffer catastrophic consequences. Experts predict that a rise in global temperatures of just a few degrees will melt polar ice, and result in a rise of sea levels enough to put many coastal cities under water. The extinction of many species of plants and animals is also predicted by some scientists. In view of these and other significant adverse effects from burning fossil fuels to generate energy, there is a significant need for a method and apparatus that can generate energy in a cost-effective manner without the generation of significant greenhouse gasses.
The present invention is directed to facilitating conversion of solar energy into useful electrical or chemical energy by processes that either require strongly concentrated solar energy, or are made more efficient or more cost effective with concentrated sunlight. In particular, the present invention is directed to an apparatus for electricity generation through photovoltaic or thermal processes. It may also be used for converting solar energy to chemical energy by thermal or photochemical reactions.
The efficiency of thermal conversion is limited by the second law of thermodynamics, which requires high temperature and hence highly concentrated light for high conversion efficiency. Photovoltaic conversion efficiency may also be improved in highly concentrated light. Thus, in the past, significant effort and attention has been given to the problem of converting solar energy at high concentration, but the results have not been altogether satisfactory. Despite the availability of solar conversion devices that operate more efficiently in highly concentrated light, high concentration solar energy systems have not yet become a major factor in global solar electricity production. A significant drawback to prior attempts at high concentration systems has been the high cost of the opto-mechanical systems required to provide highly concentrated light to solar conversion devices. Prior efforts have not been cost competitive, in part, because of the unfavorable scale sizes of the optical concentrators. These prior attempts often involved the use of particular conversion devices requiring input of concentrated sunlight at either very low levels or very high levels of power, and such power requirements led for the most part to either very small or very large optical apertures for the sunlight concentrators. This design approach failed to adequately minimize the cost per unit of generated electricity (or other form of energy).
In the past, small unit size was favored for most photovoltaic converters. This led to inefficiencies and high costs. Arrays of large numbers of small units with small optical concentrators were expensive to manufacture, assemble and transport, because they were both large and complex, with many small optical and electrical and thermal components arrayed over a large area. Also arrays of small units conferred little stiffness on large scales, and would be carried as dead weight on a two-axis solar tracker. Substantial additional weight in the form of structural members was needed to control gravity and wind bending and to make connection with the tracking mount. This design approach added significantly to the cost of a tracker to be used in such systems, and failed to achieve adequately minimized cost per unit of generated electricity (or other form of energy).
In the past, very large unit size was favored for solar thermal systems requiring very high solar power input at high concentration. Large optics concentrating in two dimensions to meet these requirements led to inefficiencies. In some systems a thermal converter unit was mounted at the focus of a single large dish, with a dish-engine assembly carried by a two-axis tracker. In other systems, a thermal converter unit was fixed on a tower with sunlight concentrated on it by a field of typically thousands of flat mirrors on heliostats. Both of these design approaches were sub-optimal in terms of minimizing the concentrator system cost per watt delivered, and failed to adequately minimize the cost per unit of generated electricity (or other form of energy).
For large single dishes, structural complexity and both mass and cost per unit area all rise with dish area. In the past, attempts were made to use a large reflector assembled from many small curved reflector segments. In a large reflector made from numerous curved reflector segments, each individual segment must be aligned on a paraboloidal back-up truss structure. A composite large reflector assembled in this manner tended to be heavy and expensive to manufacture and assemble. In many cases, large round dishes were mounted individually on two-axis trackers and set out in a large solar farms. Such large round dishes must be spaced well apart to avoid significant self shadowing in the early morning and late afternoon, resulting in sub-optimal use of land. A further disadvantage of large round dishes was relatively high wind load per unit area, which required heavier and more expensive support structure to resist wind loading.
In the past, very high power concentration was attempted by using a field of heliostats. A significant disadvantage to this approach was the inefficient use of trackers carrying a given reflector area. This inefficiency resulted because the sunlight incident on many of the heliostat mirrors was at angles far from normal incidence, thus the daily average of solar power delivered to the power tower was only a fraction of what could be captured if each mirror could be tracked to face the sun. Another disadvantage was the mechanical complexity of many small two-axis trackers for heliostats.
A further difficulty with many prior solar concentrator systems has been the special demand placed on their two-axis trackers. Compact altitude-over-azimuth mounts on a vertical pedestal have been used. Typical designs were not balanced about the elevation axis, and therefore typically suffered from high, concentrated drive loads and consequently required heavy drive mechanics. Of course, heavy drive mechanics increased the cost of such systems, and in solar energy systems, cost is a critical factor that separates success from failure. Large, high concentration dishes for thermal conversion were often especially compromised in their mechanical structure by a large radial opening for an arm to support a fluid heater, engine or turbine and electromagnetic generator at the focus.
Prior systems for using concentrated sunlight have left significant room for improvement. Solar energy systems are unlikely to have a significant impact on reducing greenhouse gasses in the atmosphere until electricity can be generated using solar energy at a cost that is competitive with electricity generated by burning fossil fuels. Cost is critical to solar energy systems. In fact, cost cannot be over emphasized, because it is so important that cost alone can make the difference between success and failure. As long as solar generated electricity costs more than electricity generated by burning fossil fuels, there is little chance that solar power is going to have a significant impact on reducing greenhouse gasses in our atmosphere. There has been a long felt need for an apparatus and method of manufacture for a solar conversion system that has a low total system cost and that is capable of generating electricity at a cost that is competitive with electricity generated by burning fossil fuels.